The present invention relates to a hollow cathode lamp used as the light source for the atomic absorption and scintillation spectroscopies.
The structure of the conventionally fabricated hollow cathode lamp will be described referring to FIGS. 1(A) and 1(B).
FIG. 1(A) shows a cross-sectional view of the internal structure of the conventional hollow cathode lamp and FIG. 1(B) shows a perspective view of the electrode structure thereof.
Bulb 2 is fastened to base 7 to form a sealed envelope. A rare gas at a pressure of the order of torrs is filled in an envelope together with the electrodes which will be described hereafter.
Light generated inside bulb 2 is radiated outside through light emission window 1 at the top of bulb 2.
Cylindrical anode 3, with the same diameter as the cathode, is connected to the external circuit through anode leads 3a--3a which are connected to anode lead pin 9 fastened to base 7, and it is supported by base 7 within a space defined by bulb 2. Most of the surface of the anode leads 3a--3a are covered with an insulator covering 6 fastened to stem 2a of a bulb 2.
Hollow cathode 4, which constitutes a cylinder with a hollow at the center thereof, is connected to cathode lead pin 8 fastened to base 7 through cathode lead 4a.
Cathode lead 4a is covered with an insulator cover forming part of stem 2a of bulb 2. Extending upwardly from the end of this insulator cover is an insulator cylinder 5 which covers the cathode 4.
When a firing voltage of 400 to 600 volts DC is fed from the external circuit to the lamp, ionized gas molecules within an envelope collide with the inner wall of the cylindrical cathode 4 so as to evaporate the cathode material. The bright line spectrum of the material forming the cathode 4 can thus be obtained.
The cathode material evaporated from cathode 4 drifts into light emission window 1 by thermal diffusion and is deposited onto light emission window 1. This reduces the transmittivity of light emission window 1. If the hollow cathode lamp is operated at high current, a substantial quantity of cathode material is evaporated from the cathode 4 and the light emission window 1 is covered with the cathode material in a short period of time.
A disk with an aperture at the center thereof, if arranged in a space between cathode 4 and light emission window 1, blocks the light emission window 1 from the cathode material and protects it against decreasing of the transmittivity. (The structure of the disk with an aperture will be described referring to preferred embodiments of the present invention.) It has been found that the structure of the disk in the conventional hollow cathode lamp is not so advantageous as expected.
The objective of the present invention is to present an improved type of hollow cathode lamp constructed to keep the light intensity unchanged by blocking the light emission window from the cathode material which reduces the transmittivity when it adheres to the light emission window.